Previous studies indicate that male rats are more sensitive to the antinociceptive effects of opioids than their female counterparts and that males display higher nociceptive thresholds. Although the gonadal hormones appear to play a critical role in mediating these sex differences, the exact nature of this role remains elusive. The present proposal will systematically evaluate the contribution of gonadal hormones to sex differences in nociception and opioid antinociception using three well-established techniques (i.e., monitoring of the estrous cycle, gonadectomy, hormone replacement therapy) and a number of innovative methodological controls. A unique component of this proposal is that it will combine the use of low-efficacy opioids known to produce profound sex differences in opioid antinociception, with the use of divergent strains of rats that differ in the magnitude of their initial sex difference in nociception and opioid antinociception. Experiment I will examine the influence of estrous cycle phase on levels of nociception and opioid antinociception. Although in each phase of the cycle gonadal hormones levels change dramatically, even when hormones are at the lowest levels they may have a profound influence on both nociception and opioid antinociception. Thus, Experiment II will examine the influence of gonadectomy in male and female rats on opioid antinociception in strains that differ in their magnitude of sex differences in opioid antinociception initially observed. Finally, Experiment III will examine the specific gonadal hormones (estrogen, progesterone, and testosterone) involved in mediating nociception and opioid antinociception in male and female rats using hormone replacement therapy. By contrasting the effects of gonadal hormones using three different procedures, controlling for type of nociceptive assay, rat strain and type of opioid, this series of investigations should provide important insight into the role of gonadal hormones in mediating nociception and opioid antinociception.